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(Pbo) Fibers Under Elevated Temperature and Humidi FACTORS CONTRIBUTING TO THE DEGRADATION OF POLY(P- PHENYLENE BENZOBISOXAZOLE) (PBO) FIBERS UNDER ELEVATED TEMPERATURE AND HUMIDITY CONDITIONS A Thesis by JOSEPH M. O’NEIL Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August 2006 Major Subject: Mechanical Engineering FACTORS CONTRIBUTING TO THE DEGRADATION OF POLY(P- PHENYLENE BENZOBISOXAZOLE) (PBO) FIBERS UNDER ELEVATED TEMPERATURE AND HUMIDITY CONDITIONS A Thesis by JOSEPH M. O’NEIL Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved by: Chair of Committee, Roger Morgan Committee Members, Jaime Grunlan Michael Bevan Head of Department, Dennis O’Neal August 2006 Major Subject: Mechanical Engineering iii ABSTRACT Factors Contributing to the Degradation of Poly(p-phenylene benzobisoxazole) (PBO) Fibers under Elevated Temperature and Humidity Conditions. (August 2006) Joseph M. O’Neil, B.S., Texas A&M University Chair of Advisory Committee: Dr. Roger J. Morgan The moisture absorption behavior of Zylon fibers was characterized in various high temperature and high humidity conditions in a controlled environment. The results of these thermal cycling tests show that PBO fibers not only absorb, but also retain moisture (approximately 0.5-3%) when exposed to elevated temperature and humidity cycles. Also, the impurities of Zylon fibers were characterized through the use of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and solid state Nuclear Magnetic Resonance (NMR). These tests demonstrated that, in addition to other impurities, PBO fibers may contain up to 0.55 weight percent phosphorus, and that this phosphorus is present in the form of phosphoric acid. It was also shown through accelerated hydrolytic degradation tests that production procedures used to neutralize the acid present in the fibers have a beneficial effect on the hydrolytic performance of the fiber. The data collected in this study was then compared and contrasted to known Kevlar studies, identifying similarities, differences, and potential trends. The results of these tests seem to indicate that there is accelerated acid catalyzed hydrolysis occurring in the fiber which is causing these fibers to degrade at an increased rate. This condition is further accelerated by heat and humidity induced permanent fiber swelling. iv ACKNOWLEDGMENTS First, I would like to thank my family and friends for their invaluable support throughout my graduate studies. I would also like to express my appreciation to my advisor Dr. Roger J. Morgan for his support and guidance from the beginning. I also wish to thank the other members of my committee, Dr. Jaime Grunlan and Dr. Michael Bevan for their support. A special thanks is due all of those people at Texas A&M who assisted my research in innumerable ways including Dr. Abraham Clearfield, Vladimir Bakhmoutov, and Dr. William D. James. In addition, I would like to thank Dr. James Zheng of the U.S. Army at Fort Belvoir and the State of Texas Advanced Research Program for allowing me to work on this interesting project. Finally, I would like to thank Cynthia Young for her assistance throughout the experimentation and research process. v TABLE OF CONTENTS Page ABSTRACT ............................................................................................... iii ACKNOWLEDGMENTS .......................................................................... iv TABLE OF CONTENTS ........................................................................... v LIST OF FIGURES .................................................................................... vii LIST OF TABLES ..................................................................................... viii CHAPTER I INTRODUCTION .......................................................................... 1 Body Armor in History ......................................................... 1 Brief History of Zylon ........................................................... 2 II THEORY AND REVIEW OF LITERATURE .............................. 4 Zylon ..................................................................................... 4 Kevlar .................................................................................... 9 III EXPERIMENTAL DETAILS ........................................................ 22 Materials ................................................................................ 22 Humidity Moisture Absorption Test Details ......................... 22 Chemical Analysis Details .................................................... 24 Solid-State NMR Details ....................................................... 24 Boiling Fiber Details ............................................................. 26 IV DATA ANALYSIS AND DISCUSSION ...................................... 27 Humidity Moisture Absorption ............................................. 27 Chemical Analysis ................................................................. 34 Solid-State NMR ................................................................... 35 Boiling Fiber Test .................................................................. 36 vi CHAPTER Page V CONCLUSIONS AND FUTURE WORK .................................... 38 REFERENCES ........................................................................................... 41 VITA .......................................................................................................... 44 vii LIST OF FIGURES FIGURE Page 1 Chemical structure of PBO ............................................................... 4 2 Synthesis of PBO .............................................................................. 5 3 Chemical structure of Kevlar ............................................................ 9 4 Amide group (left) and aromatic group (right) ................................. 10 5 Schematic of Kevlar fiber production process .................................. 11 6 Hydrolytically induced chain scission of PPTA molecule ................ 12 7 Plot of ln(ds /dt) versus 1/T for degradation of Kevlar fibers ..................... 15 8 Scanning electron micrograph of the fracture topography of a Kevlar 49-epoxy composite strand with hydrolytically degraded fibers .................................................................................................. 18 9 Typical behavior of Zylon sample during moisture absorption test .. 29 10 Four samples subjected to the same temperature-moisture cycle ..... 30 11 Moisture pickup given as percentage of original dry fiber weight ... 31 12 Experimental ratios of percentage of moisture pickup ...................... 32 13 Incremental ratios of percentage of moisture pickup ........................ 33 14 Phosphorus-31 NMR spectra ............................................................ 36 viii LIST OF TABLES TABLE Page 1 Rates of hydrolytically induced strength degradation of Kevlar 49 fiber as a function of temperature at 100% RH ................................. 14 2 Estimated Kevlar 49 strength degradation from hydrolysis .............. 16 2- 3 Ash, Na, SO4 , and S in Kevlar 49 fibers ......................................... 19 4 Chemical impurities in Kevlar 49 fibers ........................................... 20 5 Results of ICP chemical analysis showing weight percent of phosphorus ........................................................................................ 35 1 CHAPTER I INTRODUCTION Body Armor in History Throughout recorded history, humans have been in a race to create ever more and more effective weapons to use against each other. As a consequence of this, a concurrent race has been taking place to create increasingly effective protection against these weapons in the form of body armor. Some of the earliest forms of body armor were animal skins which were worn over the body. These were then supplemented or replaced by shields made out of animal hide, wood, or metal. Prior to the middle ages, humans began to wear metal armor such as plate mail or chain mail which could help protect against a blow from a sword or similar weapon. With the advent of the widespread use of firearms in the late 16th century, the push for personal ballistics protection began. While the medieval Japanese were apparently the first to use fabric (silk) as personal protection against firearms, it was not until the late 19th century that the first use of “soft body armor” was recorded in the United States. During the early 20th century body armor made from metals such as steel was being invented concurrently with the use of lighter but less effective soft body armor made from material such as cotton padding. World War II saw attention give to the “flak jackets” worn by pilots which were intended to protect against shrapnel. With the marketing of the high strength fabric Kevlar in 1971, a material was finally available which was much more suitable for use as lightweight ____________ This thesis follows the style of Journal of Composite Materials. 2 ballistics protection for both law enforcement as well as military use. This material has been highly effective in this use and remains one of the dominant materials to this day in the field of lightweight ballistics protection. Brief History of Zylon One of the most recent developments in the field of soft body armor was the marketing of the high strength fiber Zylon (PBO) in 1998. PBO is currently
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